PROJECT SUMMARY We propose to develop a strategy for understanding olfactory coding by linking the molecular identity of odorant receptors (OR) to their odor sensitivities in vivo and discovering the logic of neural circuits that process smell. Unlike for other sensory modalities (i.e. vision, audition), it is not understood what properties of odorants are important for olfaction, how these properties are processed by olfactory neural circuits, and how odorant receptor genes have evolved to optimize such an encoding. The relationship between odorant structure (chemical space), the sequence of odorant receptors, the underlying spatial-temporal patterns of activity in the brain (neuronal space) and the perceived odor quality (perceptual space) has been elusive. An efficient method for connecting the olfactory sensory spaces will have a paradigm-shifting effect on olfactory research. Our multidisciplinary approach will use cutting edge next-generation sequencing technologies (FISSEQ, MAPseq and RNAseq) together with functional widefield fluorescence and two photon imaging in vivo to define the functional properties of olfactory sensory neurons that express defined odorant receptors (ORs), to discover their connections to individual glomeruli olfactory bulb (OB), second order OB output (mitral/tufted) cells and map their projection statistics to the downstream olfactory processing brain areas. Using these tools, we will map the identity and spatial layout of all 3,500 glomeruli in the mouse olfactory bulb according to the OR types from which they receive inputs. We will further link the molecular identity of ORs to their glomerular responses to hundreds of odorants (>500) in the form of OR/odorant binding affinity matrices across hundreds of glomeruli (~500) that are optically accessible in vivo. In the same samples, we will track thousands (>1,000/experiment) of individual olfactory bulb projections to their input glomeruli and their target brain areas by RNA-barcoding in relation to their tuning to odorants via multiphoton imaging in vivo. Our approach will bridge the gap between the molecular biology of ORs and neurophysiology and will usher in a new era of understanding the functional basis of olfaction. It will allow unprecedented resolution and throughput for determining OR-ligand interactions across hundreds of odorants, and the connectivity of tens of thousands of single neurons at once in a single specimen. The data obtained will enable the study of OR- ligand interactions, relate the chemical identity of odorants to olfactory perception, and the construction of artificial nose devices for immediate biomedical applications, including disease diagnostics.